Bibliography: pages 79-86. / This thesis involved a study of the phase transfonnations in a chromium containing corrosion resistant dual phase steel, designated 3CR12. The objectives included the detennination of time-temperature-transformation (TTT) diagrams for the transformations between austenite and ferrite and an investigation into the factors controlling these reactions. The austenite decomposition reaction for a high nickel alloy, 3CR12Ni, and the effect of varying titanium concentrations on the equilibrium phases present in 3CR12, were also examined. Dilatometry was used to determine the transformation temperatures between austenite and ferrite and the Ms temperatures for the alloys investigated. The kinetics of the reactions were investigated by optical microscopy using two different etching techniques while the volume fractions of the various constituents were determined by a point counting method. Transmission electron microscopy was used to study the carbide morphologies and the nucleation and growth modes of the phases during the transformations. The distribution of the alloy elements were determined by microhardness measurements, an electronprobe microanalysis and a Kevex spectrometer attached to a scanning electron microscope. The 3CR12 alloy used in this study did not become fully austenitic above the Ae₃; it lies in the nose of the gamma loop of the Fe-Cr phase diagram. Two temperature regimes were identified on the decomposition of austenite. At 750°c the existing ferrite grains grew into the austenite matrix, while at 650° C and 700°C new ferrite was sympathetically nucleated i.e. it was heterogeneously nucleated on existing ferrite/austenite grain boundaries. Two types of carbide morphologies were formed. These were random precipitation within the ferrite grains and interphase precipitation. The TTI diagram showed conventional "C" curve kinetics. The austenitisation reaction occurred by a para-equilibrium mechanism. The rate controlling process was the structural change from ferrite to austenite; the reaction was not long range diffusion controlled. The speed of the reaction increased continuously with increasjng transformation temperature. No growth of ferrite occurred on isothermal transformation of 3CR12Ni at temperatures below the Ae₁. Increasing the bulk titanium content increased the Ms, Ae₁ and Ae₃ temperatures of 3CR12 due to the removal of carbon from, and the addition of titanium to, solution.

The life span of high temperature power plant pipework is principally a function of material creep damage - an irreversible plastic deformation of the material when subjected to temperatures and loads beyond a certain threshold. Within Eskom, the South African parastatal power utility, creep damage is primarily quantified by way of Metallographic Replication (replicas). This is a quasi NonDestructive Examination (NDE) technique that looks at the microstructure of the sample in question. Although well-known and used extensively, replicas, as with any technology, have their shortcomings. Extracting of replicas and their subsequent analysis are manual processes that inherently suffer from subjectivity. Furthermore, storage and archiving of vast quantities of physical replica slides for future reference is cumbersome - a challenge that digitisation can address. The aforementioned vulnerability to analysis subjectivity and benefits of digitisation are areas which a technology known as Digital Image Correlation (DIC) - a non-contact, full field, deformation measurement technique - can potentially address. Some research has been done on using DIC for power plant creep measurement; however literature quantifying its performance in this specific application is scarce. This study thus looks into setting up a DIC system optimised for measuring strain in an area of the pipework welds known as the Heat Affected Zone (HAZ) - the weakest part of the weldment. The achievable accuracy is established and the major parameters that affect DIC accuracy are investigated, elucidating the trade-offs between optimising each. In addition, two scenarios exist for the acquisition of DIC measurement data from a plant: when the plant is operating (online); or when the plant is shut down for maintenance (offline). The encumbrance of imaging a hot surface makes the former scenario the more demanding, and was thus investigated. This data was subsequently used to substantiate whether DIC has the potential to be used online (i.e. at elevated temperatures) or is limited to use during shut downs.

This thesis investigates the influence of nitrogen concentration on the deformation behaviour of a modified AISI 200 type alloy. The Fe-18Cr-4Ni-7Mn base alloys contained a range of nitrogen contents from 0 to 0.27 wt%. The tensile behaviour was characterised by uniaxial tensile testing. The room temperature yield strength was shown to decrease with increasing nitrogen content for nitrogen contents less than 0.1 wt%. This decrease was attributed to the presence of secondary phases in the annealed state. An increase in yield strength was observed with further additions of nitrogen and this is probably due to solid solution strengthening and a Cottrell interaction. The variation of room temperature ultimate tensile strength showed no dependence on nitrogen content. Elevated temperature tensile tests (120DC) showed an increase in ultimate tensile strength and yield strength with nitrogen content suggesting that solid solution strengthening and a Cottrell interaction are occurring in this alloy range. Room temperature deformation was shown to induce a transformation from austenite to martensite throughout this alloy range, the degree of transformation decreasing with increasing nitrogen content. This transformation was shown to provide considerable strengthening and work hardening to the alloy. The low stacking fault energy (SFE) of this alloy range ensured that cross-slip was significantly inhibited during room temperature deformation and the deformation mode was observed to be planar glide. Nitrogen was shown to increase the SFE of this alloy range thus causing a decrease in the inhibition of cross-slip with increasing nitrogen content. Increasing the nitrogen content thus causes a decrease in strength if the contributions of a deformation-induced transformation, SFE variation and secondary phases are considered but causes an increased strengthening contribution due to solid solution strengthening and a Cottrell interaction. Hence as the nitrogen content increases different strengthening mechanisms are activated and no overall trend of room temperature ultimate tensile strength with nitrogen content is observed. The ductility and formability of the steels did not appear to show any distinct trends with changes in nitrogen content.

Includes bibliographical references. / The aim of this work was to investigate changes in the electronic structure of platinum as a result of alloying with vanadium, and the effects of these changes on O2 adsorption. This is important for the further development of hydrogen fuel cells, because the oxygen reduction reaction (ORR) presently requires O2 adsorption to occur on pure platinum, which is a prohibitively expensive material. A computational study has therefore been undertaken on alloying platinum (which reduces cost) with vanadium (for which there is plentiful experimental data) and the consequences for O2 adsorption. The first moment of the d-band of platinum alloy DOS was used to represent the d-band centre. The d-band centre of Pt-PDOS became lower as a result of hybridisation between platinum and vanadium. The d-band centre of a pure platinum surface with respect to the Fermi level is -1.99eV, but it is shifted to -3.23eV when vanadium atoms are added to the subsurface layer. The adsorption energies of O2 are sensitive to a combination of calculation parameters used. In this work, the calculations were executed using the CASTEP code. This is a plane wave pseudo potential code. The most stabilised geometry of an adsorbed molecule on pure Pt (111) was at the fcc site and had an adsorption energy of -1,91eV. The adsorption energy at the bridge site of Pt (111) is -1.81eV. When subsurface vanadium atoms were introduced, the equilibrium surface-molecule bond lengths increased. The adsorption energy at the fcc site shifted to -1.37eV, -1.43 for the bridge site and -1.45eV for the hcp site. It was concluded that the presence of vanadium atoms in the surface region destabilises an adsorbed oxygen molecule but a more detailed study is needed to show the effect of the solute atoms on the thermodynamics and kinetics of the whole oxygen reduction reaction chain.

Bibliography: pages 90-97. / The new concept of hydropower has been found to be technically feasible in South African gold mines. Chilled mine service water is piped from the surface to deep level stope; where the hydrostatic pressure provides power for stoping machinery. This water varies widely in composition and acidity. High concentrations of sulphate, chloride and nitrate are present. These ions are derived from the leaching of oxidised sulphides from the broken rock, the fissure water and the dissolution of blasting fumes. In order to minimise the deterioration of stoping machinery by corrosion and synergistic corrosive abrasive effects, a compromise between selecting a suitable corrosion resistant material and treating the mine service water to an acceptable level of corrosiveness is being sought.

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Cartilage tissue engineering (TE) is a promising osteoarthritis therapy whereby cell-seeded constructs are generated in vitro for use in restoring degenerated cartilage in patients. While cartilage TE technology has exhibited growing clinical success, it continues to be encumbered by the utilization of high cost and laborious protocols, such as the need for frequent replenishment of culture media (every other day) during the duration of standard in vitro cultivation phases (2-8 weeks). This constitutes a significant time/cost burden for researchers and clinical technicians. Interestingly, the adoption of this convention is based on traditional cell culture protocols, rather than on a fundamental understanding of the stability of culture media constituents in current cartilage TE culture systems, leading one to consider that current TE replenishment protocols may be far from optimized. In the current study, we hypothesize that larger media volumes can be used to: 1) mitigate the depletion of constituents and accumulation of waste products in tissue constructs over time and accordingly, 2) reduce the media replenishment frequency required to generate engineered cartilage with functional mechanical properties and composition. Bovine chondrocyte-seeded agarose constructs (Ø4mm×2mm) were cultivated for 7 weeks in chondrogenic media of increasing cumulative media volumes (3mL, 6mL, 9mL, 18mL, and 54mL) and replenishment frequencies, including the conventionally utilized thrice-weekly and lowered frequencies of weekly, biweekly, and replenishment-free. The stability of influential media constituents (glucose, ascorbic acid, insulin), waste product accumulation (assessed via pH), and the properties of constructs were monitored over time. Results demonstrated that concentrations of growth-promoting media constituents and pH decreased over culture duration but this decrease can be mitigated by the use of larger replenished media volumes. For all replenishment frequencies, tissue construct mechanical properties and sulfate glycosaminoglycan (sGAG) content generally increased with replenished media volumes. For weekly, biweekly, and replenishment-free frequencies, the generation of constructs with native properties required the higher replenished media volumes per replenishment but did not require the use of higher cumulative media volumes. These results suggest that functional engineered cartilage can be generated with lower media replenishment frequencies or replenishment-free conditions. These protocols may be adopted in clinical and research-grade TE platforms to reduce labor costs and contamination risk. / 2023-09-24T00:00:00Z

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The narrow band gap semiconductor iron antimonide, FeSb2, has been grown as bulk single crystals, and investigated for its electronic and magnetic properties at low temperatures. The electronic behavior of FeSb2 is associated with strong electron correlations, similar to the characteristic behavior observed in FeSi, another strongly correlated d-electron correlated semiconductor. Recent studies found an enhancement of the Seebeck coefficient and thermopower, two key parameters in thermoelectric performance, at temperatures below 77K, making FeSb2 potentially useful in cryogenic Peltier-cooling applications below liquid nitrogen temperatures. Crystals with sizes of 1.5-5.0 mm are grown using three different synthesis approaches: chemical vapor-phase transport (CVT), using either chlorine (Cl2(g)) or iodine (I2(g)) as transporting agents, and molten flux growth using excess antimony (Sb). Single crystal and powder x-ray diffraction experiments on select samples provided for structural phase identification and confirmed the absence of secondary impurity phases. FeSb2 crystallizes in the FeS2-marcasite structure type, featuring FeSb6 edge-sharing octahedra forming chains along the c-axis and corner-sharing in the a-b plane. The crystal surfaces were characterized using SEM-EDS and AFM measurements, revealing the characteristic morphological features resulting from CVT growth, as well as helping to identify surface contamination. Magnetic susceptibility measurements show weak temperature induced paramagnetism above 50K, with the diamagnetic-to-paramagnetic crossover above 100K. Temperature-dependent electronic transport properties, ρ(T), showed semiconducting behavior and the formation of a resistivity plateau between 10K-40K, corresponding to a secondary transport gap, εg, of about 3.9-7.2meV, similar in magnitude and in accordance to previous studies on FeSb2. Hybridization between iron (Fe) 3d and antimony (Sb) 5p and 5s valence states appears to be responsible for complex electronic behavior at low temperatures, with the formation of a small secondary gap. Earlier studies have shown a significant enhancement of the Seebeck coefficient (thermopower) at the onset of this gap (10K-12K). Thermopower measurements in our sample also show peak values around this temperature, but the maximum values are several orders of magnitude lower than previous reports, while magnetic and electronic properties are in good agreement. / A Thesis submitted to the Interdisciplinary Program in Materials Science in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester, 2012. / May 22, 2012. / Correlated Systems, FeSb2, FeSi, Peltier cooling, Seebeck Coefficient, Thermoelectrics / Includes bibliographical references. / Theo Siegrist, Professor Directing Thesis; Eric Hellstrom, Committee Member; James Brooks, Committee Member.